It is common practice to couple light from an optical waveguide such as an optical fibre to one or more optical fibres via an at least partially transmissive, partially reflective optical element. For example a dichroic filter is often disposed between a pair of collimating/focusing lenses such as graded index (GRIN) lenses to provide a wavelength division multiplexing (WDM) function. FIG. 1 illustrates a prior art WDM filter, wherein wavelengths of light .lambda.1 and .lambda.2 are launched into a first port at an end face of a GRIN lens 12a and wherein light of wavelength .lambda.1 is reflected from a filter 14 which passes light of wavelength .lambda.2. The lines indicating the path of the beam as it is partially reflected and partially transmitted through the filter represents a ray through the centre of the beam launched into the lens 12a. FIGS. 2 and 3 illustrate the beam's shape and path for light launched into a port along the optical axis of the lens and for light launched into a port offset from the optical axis respectively.
Prior art FIG. 4 illustrates a typical arrangement of one side of a WDM filter wherein an optical fibre tube 8 holding two optical fibres at predetermined locations is shown optically coupled with an adjacent GRIN lens 12c which is juxtaposed to a filter 14. Fibre tubes or sleeves of this type provide a convenient way of holding two or more optical fibres a fixed distance apart such that the pair of fibres can then be moved without damaging them. Furthermore, the tube allows the fiber ends (and tube) to be polished such that the end face is slanted to lessen the effects of unwanted back reflections.
Heretofore, there has been no known reason to align pairs of optical fibres at the slanted, polished end face of the sleeve in any special orientation except with regard to pairs of optical fibres being offset a same distance from the optical axis. The term pair relates to two optical fibres which via some reflective element are optically coupled to one another.
Prior art FIG. 13 illustrates pairs of optical fibres (A,B), (C,D), and (E,F) disposed at a slanted end face of a fibre tube 8. By placing a reflective element at an appropriate location and position, and by providing an appropriate collimating/focusing lens, light from one of each of the pairs will couple into the other of the pair in an expected fashion.
What is unexpected, is that if the fibres are located at any locations other than along one particular line through the optical axis, coupling will be negatively affected. For example the positioning of the pair of optical fibres shown in FIG. 5 is least favourable for optimum coupling in a WDM filter. This is illustrated in more detail in FIG. 6, where the path AC and the path CB are of different lengths illustrated by .DELTA.L due to the position of the fibres 13a and 13b on the slanted end face of the sleeve 8.
FIG. 7 more clearly illustrates the problem with haphazardly disposing the fibre equidistant from the longitudinal axis of the sleeve or more importantly from a line extending from the optical axis of the lens 12d wherein a beam leaving the output fibre at point A is perfectly collimated at the reflective surface 16 through point C, and wherein the reflection from 16 is focused at point D along the dashed line 18, instead of being focused at point B coincident with the optical fibre end.
It is an object of this invention to provide an arrangement of optical fibres within a tube or sleeve that will couple light in an optimum manner.
It is a further object of this invention to provide an optical system wherein a preferred coupling is achieved with essentially no additional components from previous similar coupling methods.